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When cruising, is any moment that a pilot will reduce his/her airplane speed?
Say, a pilot is piloting his/her jet commercial airplane and suddenly see cumolonimbus or a dark cloud. Then, should the pilot reduce his/her airplane's speed to avoid turbulence?

What you're worried about is hitting turbulence. The word "impact" isn't typically used, although hail impacting an airplane has on relatively rare occasions damaged airplanes in flight and even more rarely has brought them down. My experience was that it just makes a lot of noise on the windshield.
Your question title is more inclusive than the body. Taking the narrower question in the body first— yes. If a pilot is expecting or experiencing severe turbulence, he should lower the cruising speed if that speed is above the recommended turbulence penetration speed. And in lesser degrees of turbulence, some might choose to lessen the speed if they believe that doing so will give passengers a less uncomfortable ride.
Now as to "any moment" and interpreting that as "any time", there are a few instances I can think of offhand.

I knew a few pilots that, when they were paid by the hour, would sometimes reduce speed, although I think that they would usually just not bring the aircraft up to the normal cruising speed rather than get that speed and then slow. To me that was unprofessional. I flew with one such captain a few times, and on my legs if I put the airplane (747-100/200) at to mach 0.86 for cruise, he'd ask for it to be slowed.
In specialized and rare instances, it might be desirable to reduce your speed to be a little more fuel efficient. For example, JFK to Tel Aviv in a 747-100 would usually require a redispatch at a fix abeam Athens. To be legal you had to have a certain minimum fuel at the redispatch point before proceeding to Tel Aviv, otherwise you'd have to land at Athens for fuel. So, if approaching the redispatch point — say an hour or two away from it — you might choose to slow to have a little more fuel on arrival at the redispatch point if you thought not doing so would result in a little less fuel than required.
Then there's the instance where you know you're going to be thrown into a hold on arrival at the destination so you may as well slow a little getting there, burn a little less fuel, have to hold a shorter time. You have to be careful with this one though, because it might mean others get into the hold sooner than you would have, and all you had done is put yourself farther back in the line than you would have.
And, of course, ATC may ask for a cruise speed reduction to minimize excessive flow at the destination.

Thank you Sir your for your explanation. The body just an example case I use to describe my question.

The word "turbulence" is the most suitable. I tried to remember what is the word mostly mentioned by pilots related to the cumolonimbus every time got airplane accident in the air when there is tv interview. Apologize for the lag of my knowledge.

As long as you don't cause a late arrival, there's nothing wrong with slowing down cruise.  Some companies encourage it, and often times arriving early just means you have to sit and wait for a gate.  Better to slow down and save some gas, and yes, sometimes get paid more.


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Is smart phone's GPS accurate enough to calculate airplane speed?
Around seven years ago, I did airplane speed measurement. I activated the GPS on my tablet and I monitored the speed, especially when the airplane touched the ground (runway). I found that the airplane touched the ground with speed 420kph, which it was fast enough. The measurement was probably not 100% accurate, but I could feel that the airplane was landing and touched the ground quite fast.
My question, how accurate is measurement with GPS, especially using GPS installed on smart phone? If I need to calibrate, how to calibrate?
Edit:
The airplane was Boeing 737. Not sure the series.

Is this really an aviation question, though? To me this reads as: "how accurate are GPS speed measurements?" which can be answered by a google search. The only cases where aviation would play a part in this is at altitudes where GPS hardware is inhibited or if you were asking if the precision is enough for some specific case.

This should be aviation as the context is in aviation. For example, how to calibrate due to the airline speed, etc..

I don't see how this is about aviation. Yes, you say that "the context is in aviation", but the question is squarely about the GPS functionality of an unspecified tablet from back in 2011-ish. The exact same question can be asked without any mention of the aviation aspects with no loss of answerability; ergo the question is not about aviation. Now, if the question had been something along the lines of I checked the landing speed of some B737 on my tablet's GPS in 2011, and it showed as 420 km/h, is that reasonable?, that would be an aviation question. But that's not what you're asking.

@aCVn, I was probably wrong in wording my question. But however, airplane  is also using GPS in flight. The different to my case is only the device: mine is tablet while the airplane is using dedicated device.

Actually, I would argue that this question is well within the scope of aviation.  First, the question is about measuring the ground speed of an aircraft. Second it is about using GPS, which is now ubiquitous in aviation. Third, and perhaps most importantly it is about errors in the application of GPS, which are common, and the topic of substantial debate, often by poorly informed people talking about their experiences.  Asking about calibration is a good question, unfortunately with the answer of you can't. The OP misunderstands many aspects of his question, but that is why we have questions.

It boils down to methodology used within the GPS receiver and processor.  From general navigation perspective, GPS can provide extremely accurate velocity vectors.  I'll spare the details as to why.  But as an example, it can commonly be an order of magnitude more accurate than radar or the use of common ground based navaids.
However, if the GPS receiver and processor is designed to provide successive position information then the error budget goes up.  The following factors come into play:
First, the position accuracy of GPS is not as accurate as the velocity information which could be available, and these errors are compounded with successive position determinations which might be used by the phone/tablet/device processor to determine an approximation of the velocity vector.
Secondly, the determination of velocity information with successive position processing may not utilize metadata which would be utilized with a dedicated navigation processor which has the goal of reliable velocity information.  This means an increase in the error budget.
Thirdly, navigation devices which utilize GPS for aircraft, munitions and other applications, tend to perform continuous predictions as to the expected velocity, position, etc. in the future, so that the instantaneous readings from the device are a combination of prediction and historical data.  In a tablet or smartphone, without knowing the attributes of the GPS and nav processor(s) one is likely looking at historical data, without the benefit of predictive models.  This also increased the error budget.
So applied to the OP example, it is very conceivable that a reading inside the Faraday cage of a commercial aircraft, with limited sky view, multipath, and limited predictive modeling, and limited integration of sporadic readings, and without the benefit of strong predictive modeling, one may get readings that are substantially off from reality.
However, if one were using a GPS system designed for navigation, which exploited multi state Kalman filtering for velocity and position data, then I would expect a very highly accurate reading of velocity information.  In the OP scenario, all of this does not exist.
So in the OP scenario, I would not expect a high level of accuracy and repeatability, and unfortunately, with the location/antenna/receiver/processor being used there is little that can be done to enhance the accuracy by calibration.
Addendum: One additional note.  I don't know the current restrictions, but most GPS units have "speed restrictions" whereby they will not provide data above certain speeds (like Mach level speeds) which is done to limit their application to munitions.  That may not be a factor in this case, but it is worth noting that all commercial GPS processors have this built in, if they are of US origin.

@Fattie, if it's involved we can go to chat, but I have developed nav systems on and off for aerospace, and my company has provided content for every GPS payload to date.  If you want to get technical we can, but I tried to put my answer so it addressed the concerns of the OP. I have worked on other nav, including sat, aircraft, missile, etc.  Rather than vote down, let's make it right. I can change my answer or delete it if appropriate.

@Fattie, yes one could consider it sporadic performance.  But seriously, GPS is the cheapest way to get velocity information and it is extremely accurate.  If not in a Faraday plane, and with reasonable processing, it is very very accurate.

Anecdotal, but recently a friend and I used several different smart phones, including an iPhone 4S, a Motorola X 2nd Gen and a Samsung A tablet, non of which are "current" products, and checked out the GPS in the cockpit of various aircraft, also using a utility which showed the views of satellites. In the front office, the signals were a bit better, and at speeds between 300 and 450 k, we found that the inexpensive GPS displayed the steady state velocity within 2k of the FMS readout.  Altitude readings were generally low.

That landing speed sounds rather high.  There's a regulatory maximum landing speed for airliner type aircraft, and as I recall, it's typically not far from 150 kt (240 or so km/hr).  Since it's unlikely the airplane you were in was actually flying at more than double normal speed at landing (and 180 kph will seem pretty fast to your own senses) I'm inclined to believe there was an error in your measurement.
The GPS in a tablet, reading from inside the airplane, is subject to a number of error factors.  First, even in good reading conditions, the tablet's position is generally only good to (in my experience) around 50 m; second, from inside a metal fuselage, there may be path-length errors, satellite changes, and other errors on top of the accuracy limitation of your device.
This level of positional error can add up to a large percentage error in calculated speed.
The GPS installed in aircraft have better software and can fix location more accurately, as well as more regular position checks which give more accurate speed calculation.  Those instruments make a good check against the pitot/static airspeed system.  A smartphone or tablet GPS, probably not so much.

There are definitely some puzzles here:  (1) A cheap two dollar GPS is , absolutely, perfectly cable of exactly measuring speeds around "420 kph" with total precision   (2) I've always found that reception inside airliners is nonexistent or rubbish  (3) if a GPS is getting weak reception, not enough sats seen, it won't show "wildly wrong figures" - it will just state "I can't see enough satellites".  So, it's totally weird that the OP reports "420 kph" IMO.  I do not agree that path-length errors / etc could result in a "meaningful bad reading" (so, one that's out by a factor of two).

I do not agree with the final paragraph.  The  advantage is (spectacularly) better antennas, viz a viz a phone gps held by the user in the cabin.

@Fattie It probably also has to do with what app is being used to read the GPS hardware in the device.  My phone will routinely mis-estimate the distance to an intersection by a factor of three or four (either too far or too close), and works significantly better out of the car than in.  This might be different if I ran dedicated GPS app instead of Google Maps.

I guessed, the problem with the measurement was due to the airplane speed and the delay of the GPS processing, plus transmit and receive time of the satellite. We know, geostationer satellite is at 37,000km. Required at least 1.2s to travell up and down the signal, plus delay processing inside the tablet. So, when we see the reading at the tablet, it was not the actual. We know, I second is quite significant for an airplabe speed calculation. That what I am asking about calibration.

Correction to the sentence above: We know, 1 second is quite significant for an airplane speed calculation.

@AirCraftLover GPS aren't at GEO -- they're in polar orbits at (IIRC) around 250 km.  Signal propagation time is calculated in by the device (they even take into account gravitational and velocity time dilation effects).  Sampling frequency is a real potential source of error, however.

Oh I see. I thought it geo. So the delay is smaller, probably is several ms only.

@AirCraftLover And the delay is exactly what's used to calculate position.

I mean, during landing, airplane speed reduced quite fast. While device transmitted its position and satellite calculated and sent back, the airplane speed was changed. In my opinion, the measurement become not accurate as what we read was the past airplane's position. A bit realtime delay measurement.

If you read an average speed over significant time, you might have included some period when you were cruising at appr. 850-900 kph...

"There's a regulatory maximum landing speed for airliner type aircraft, and as I recall, it's not much if any over 100 kt" No, actually every single airliner I know lands at speeds in excess of 100 kt (typically around 120-160 kt depending on aircraft type, load and flap settings).

@Bianfable Yep, you're right -- I was recalling incorrectly.  Editing.

@AirCraftLover: GPS is one-way. There is no transmission of position from a GPS receiver to a satellite.

@Transistor, if GPS is one-way, so how the GPS know the position change?

The receiver calculates position on a timed cycle, multiple times a second to every couple seconds, depending on settings and application.  This lets it update position in what looks to a human like "real time", and often it will calculate velocity on the same cycle.

@AirCraft: Have a look at How does GPS work to gain some understanding. You will also find many YouTube articles.

Measuring the speed of an aircraft with a GPS has several problems, and it's suitability will depend on what you are doing with that information.
A GPS typically uses a estimation and filtering algorithm to calculate position, time and velocity.  A lesser known byproduct of these filtering algorithms are confidence intervals for each of the calculated parameters.  Generally these are not shown to users unless they are looking at a diagnostics screen, and then, not always.  Here I am talking about confidence of estimated state variables, such as position and speed.
These filtering algorithms are usually "tuned" to a dynamic model in order to intelligently weight historical measurements with current measurements.  A GPS tuned for an oceangoing vessel can count on very small accelerations and altitudes near the geoid.  I've seen models ranging from non-moving stations to airborne applications with up to 4g of acceleration.  A smartphone is probably tuned for a dynamic model of an automobile, which can be expected to have dynamics < 1g.  One consequence of this tuning is a trade-off between the noise and the latency of the output.  Latency can also be variable and may be several seconds behind the actual motion of the receiver.
Dilution of Precision (DOP) is a useful metric, (distinct from a confidence interval on a state variable).  DOP's come in several flavors and are a product of the geometry of the satellites relative to the receiver.  DOP's are improved when the angles between satellites are as large as possible.
Some answers have mentioned numbers of satellites.  In your case, your receiver's view was likely limited to satellites it could see out your, and perhaps a neighboring window, limiting the angles to a single side of the aircraft and probably less than a quarter sphere.  Your DOP is poor in this situation as compared to having open sky above you.
One thing you have going for you is that speed for GPS is essentially derived from successive position solutions.  While this makes the information inherently latent, it has the advantage of not being affected by steady position biases.
I suppose I'll end my answer by circling back to my opening statement:  What are you hoping to use the speed information for?

In cruising flight, your phone's solution will be very good for indicating your speed over the ground.
Your question mentioned measuring landing: expect several seconds of latency if you are curious about speed at touchdown.
Another question may deal with airspeed, for which your phone's GPS cannot help you.  GPS is often combined with an airdata system for a more complete navigation solution which may include indicated airspeed, true airspeed, wind speed, density and pressure altitudes, and other parameters used in aircraft control and performance calculations.

Scott Kinder, a 737 pilot, answered this on Quora:

No way to give you a precise answer. Every aircraft lands at a different indicated speed based on flaps setting, their weight, and what the pressure altitude of the airport is. Also throw in wind corrections we make to the final approach speeds.
So say a normal 30 flap landing, with calm winds. My approach speed may be 150 knots (173 mph). At sea level, indicated airspeed is close to true airspeed, that means when I touch down it’s around 173 mph. Now throw me up to Denver International. At that altitude, my 150 knots approach speed is actually a TAS of 166 knots, so now I’m touching down around 190 mph.

173 mph = 278 kph
190 mph = 305 kph
S0 420 kph seems on the high side (260mph)
Tablet speed is questionable. Did you look to see how many GPS satellites it was receiving at the time? The external GPS antenna on the plane would see ~10 when near the ground (mine on the wing of my small plane sees 9-10 most of the time).

If I was not wrong, there was 5 (as far as can I remember) satellites with good signal, probably more than 75% of the bar. 5 satellites with no other bad signal satellites. The weather at that times was clear sky.

I believe there was delay problem as the airplane is quite fast while the satellite signal had to travel two times, transmit and receive at the geo-stationer altitude, plus the processing time. And the calibration I am asking here is due to the delay.

5? Thru an airline seat window? Wow.  I thought you might have picked up 2 or 3 if you were lucky. Even with delay, most GPSs will update once a second. Aircraft (in the US) are prohibited from flying over 250mph below 10,000 feet, and then fly the descent for landing even slower, so I don't think delay is the problem there.

Yes, it is true. I was sitting in the right side, slightly in front of the wing. If at that time I understood any airplane and aerodynamic, I will do better measurement.

I believe the airplane was fast enough as the "touching" was quite not smooth. Actually, that was my reason as I observed many times that most airliner with cheap ticket, the landing almost never smooth. Quite different when I traveled overseas with good reputation airliner.

"Airline with cheap ticket" may be over weight too, hence fast landing speed.  Landing too fast can make aircraft bounce one or twice before settling, but is better than stalling an overloaded aircraft on final approach.

GPS speed can be less accurate than position.  You can of course calculate speed from changing position and that allows a certain level of averaging, but you don't know what the device is doing.  Similarly you don't know how good its report of signal strength is. Finally for a given number of satellite you get better accuracy if the bearings to them are diverse - you may have been seeing quite a few in two small opposite regions of the sky

5 is all wrong, unless you hold the phone right up to the window and you're lucky.  Also GPS is pass or fail, it either gives an awesomely-accurate value, or, it simply states "I do not see enough satellites to give you a value."  Something is fishy with the whole set of information here.

hi @ChrisH , surprisingly it is generally easier to give accurate speed than position (you can google a zillion explanations of this factoid about GPS).

@RobertDiGiovanni, that was my experience at 3 January 2007. I remember the date as 1 January 2007, there was one airplane (Adam Air) crashed into a sea, 2km deep. Only the blackbox can be taken by the US Navy using robot due to it deep. That was the deepest sea in the world. At my flight, there was bouncing when landing, which I believed every hard landing will give more stress to the structure which could make damage and accident later. But GPS was not so advanve at that time, so I could not measure. And October 2011 I did the measurement.

@Fattie, I was not new to GPS. I was a surveyor, which always work with GPS to take coordinates and altitude meter to measure locations. Clearly, I understood what I did when I did the measurement. The problem that time was I did't have any idea about aviation.

that's totally reasonable @AirCraftLover.  Which GPS software was it exactly on which phone?  A lot of phone GPS software (some made by folks I know, heh!) is total garbage, the numbers are unreadable etc.

@Fattie, that's also possible, but we don't know whether the tablet is reading GPS speed or calculating from position (I've seen apps that definitely do the latter). It's also not clear from what I've read how the speed measurement degrades with a poor signal, especially in the case I hypothesise where the bearings to the only satellites seen are roughly perpendicular to the direction of travel.

all true @ChrisH - it sounds like it happened 7 yrs past, so really, i think it would be difficult to tease out much info now  :/

@Fattie you're right. I can only point out problems, not a solution.

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How does a pilot maintain stability for an unbalance loaded airplane?
Consider Cessna 172 which has 4 seats, including the pilot, with the seat configuration as the picture below. Say the airplane has no passenger, only the pilot him/her self. I am not sure where it center gravity (CG). For longitudinal (roll), probably we can consider it separate between left and right seats. But for pitch, I don't have any idea.
My question is, if a pilot want to fly his/her airplane, how he/she assign stability control so the airplane will not has tendencies, either regarding the longitudinal (roll) or the pitch. What else control he/she needs to apply to make the airplane safely fly due to the unbalance load?

For front to rear center of gravity, longitudinal loading in other words, always try to load heaviest forward/lightest aft.  If you have a fat slob with you and a small person, put the small person in the back seat and the fatso beside you.  Don't load two in the back with the front seat empty unless you work out the CoG calculations and determine you are in limits.  If you are in the certified range the airplane will handle fine and is flown normally.
Laterally, not a lot of options and the effects of lateral imbalance are pretty mild if the fuel levels are more or less equal in a 172, so not too big a deal.

The 172 has four seats, but the rear ones are only really useful for children or baggage. Three adults plus full fuel can easily put you at/above max weight, regardless of CG.

Flying with full fuel all the time is kind of a renter's luxury.  I've flown 172s with four adults aboard and half tanks or so and they fit fine.  When I flew a 185 on floats in the bush, I was always going with less than half tanks, generally fuel for the trip plus 10 gallons or so, and rarely more than 30-35 imp gallons total, so I could take a full cabin load. In the commercial world any more fuel than necessary to get there and back, with a safety reserve, is ballast.

I can see it's a luxury it you rent wet, but renting dry only works if everyone has to top off at return. The "ballast" only really matters if it puts you over max weight, and I'd look at moving to a bigger plane rather than give up options if something goes wrong in the air. That's the real renter's luxury: using the best plane for each mission.

Dear @JohnK, I quote this: For longitudinal loading, always try to load heaviest forward and lightest aft. If you have a fat slob with you and a small person, put the small person in the back seat and the fatso beside you. Is any the more technically explanation? Don't we need to consider its traverse/pitch load? In my opinion, should be heavier aft as we have flap. Of course we have slat, but flab is look longer.

You don't want the center of gravity too far aft.  You just load to keep the CofG in the certified range, if you have the luxury of doing calculations.  If you observe the light/aft rule, you can be sure you will be in the range if you are unable to do a weight and balance calculation, which happens quite a lot in bush flying.

Thank you @JohnK for the nice explanation.

Use the available trim (elevator, and rudder if installed), and then manage the left/right fuel tanks if they are not feeding evenly.  Use the autopilot if so equipped, even if it's just a single axis wing leveler.
With just 1 passenger, it is hardly out of balance tho. The POH has a Weight & Balance chart as seen here  How are the limits of the center of gravity chart established?, plotting front seat weight and fuel will show the plane to be well within limits.

Also, a single pilot would typically sit front & left in a C172 as that is where the primary panel instruments are usually located, along with useful things like the key to start the airplane.

You will also adjust trim from time to time, as airspeed changes, fuel is burned, &c.  The idea is basically to trim so that the aircraft maintains whatever attitude you've set it at with no control input.

Dear @CrossRoads, I quote this "a single pilot would typically sit front & left in a C172 as that is where the primary panel instruments are usually located." But that position is just mirroring the situation of the seat depiction, not fully related to the balancing effort.

Dear @jamesqf, yes, there have to be done. That also must be done to a parachute jump airplane which the load is changed drastically after the passenger have jumped.

He'll have to keep some control input to keep the aircraft flying straight and level depending on how severe the imbalance is.
If there are trim controls, those can be used to help, alleviating the need to provide input manually (the C172 has only elevator trim btw., no rudder or aileron trim).

But what I need actually is a specific action to be done when facing such situation. Of course, when a pilot entering his/her plane, or even before entering his/her plane, he/she should have known the load. Thence the pilot decided what to do, even when it still ramp. It required to avoid tendencies.

I don't do any specific thing in my plane. In my plane, the stabilator  trim (vs elevator in a 172) is still where it was set from my previous landing.  When I take off, I may adjust it for a little more up-trim or down-trim as needed. You can't tell how much until you are climbing out and determine how much you are pulling on the yoke to hold the deck angle you want. If just a short climb to pattern altitude for takeoff/landing practice, maybe very little change. If going up for a longer flight, maybe a different deck angle for faster speed/shallower climb. There is no hard & fast rule.

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What is the best aircraft engine to be used on small aircraft?
I intend to build small aircraft as I explained here. But I am quite confuse to chose the BEST engine to be used. My requirement is range from 60HP to 100HP. BEST in here is considering the price, the weight/hp, the easy maintenance, and everything. By comparing all, we will get what is the best one to be used.

Apologize for such mistake. My intention actually was to get some comment regarding the brand I mentioned. I am interested as not necessary to buy completely as I myself can get many of the engine part even in my town. In my mind, when we talking about aircraft, non technical also we need to be discussed, to get some opinion. But if it is no allowed, I will not ask such it anymore. And the question now I have changed totally.

unfortunately I think that your "best" definition is too fuzzy for this site, and leaves the question as too broad.

Main issues with auto conversions:
They are run very hard compared to aircraft engines.  The same engine in the car runs at 20% power cruising on the highway.  This application is equivalent to pulling a trailer in a car that loads the engine to 75% rated power at its torque peak all the time.  However, that doesn't disqualify it if it has a track record.  Subarus are particularly robust in this regard.
Reduction drive torsional resonance.  It's been the bane of auto conversions for many decades and most of the solutions are only partial ones.  The engine has to demonstrate no torsional vibration modes and that the drive system is robust enough to trust with your life.  It's a major point-of-failure that you avoid with direct drive.
Fuel system.  I would prefer a regular carb myself.  If FI, it MUST have a fully redundant electronics package, with no single points of failure, because with solid state electronics they are either working or not and there is no degradation to warn of potential failure.
Same with ignition. I can live with single plugs, but the electronics must be fully redundant with no electronic single points of failure.
There are a lot of other details, like the crank. Is it a forged crank with properly radiused journals for stress crack resistance?
The great thing about "old fashioned" aircraft engines is that they are very "undertuned" and understressed, and will still make power with shocking amounts of internal degradation and damage.
I would think of that engine as being equivalent to an O-200 (in spite of the claim of 117 hp) which is about 18 pounds lighter, with dual redundant ignition and a simple reliable, carburator, no reduction drive failure mode, no cooling system failure mode, etc. etc.
If it was me, for that kind of money I'd hunt for a good mid time O-200.  At any rate I would want to see a lot of hours accumulated on the auto based engine to provide some kind of probability comfort zone.
Another really good alternative engine option is a Corvair. Flat 6, air cooled, carburetor, semi-dual ignition and can be modified to dual plugs, silky smooth with that awesome flat 6 sound.  They are very robust and do not run at stress levels much more than the original car application.
If it was my project I'd be going with either a used O-200 or a Corvair.

Thank you @john-k. Your comment help a lot.

If you are interested in looking into Corvairs, the guru to look up is William Wynne in Florida.  Cheers.

Sure my friend. I do appreciate your info. I will some research regarding the two brand.


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Is it possible to obtain a PPL after having a brain surgery?
I am a 24 year old who has always aspired to become a pilot. I've decided to eventually get a private pilot's license so I can fly just for fun. However, a few months ago I was diagnosed with brain tumor which I had removed. There were no complications, I have never had any seizures, I'm not epileptic, etc. As far as I am concerned (and my doctors as well), I am now in perfect health.
With this in mind, is it still possible for me to obtain a PPL?

With some aircraft types,I'd say that brain surgery is a pre-requisite!  Seriously, good luck.  You need a medical anyway and the examining doctor will make that decision or take it higher for you.  My advice is to get the medical done first; you need it to solo anyway.  I'm sure it will be fine.

Update : I have my Class 1 with an OML restriction. But this restriction may be removed 3 years after the surgery if no complications occur. And I had my CPL(A) by the way :)

According to AOPA's medical guru on their discussion board (members only, or I would link to an example) it is possible to get a medical certificate following a tumor removal but only after a 5-year wait and a battery of tests.
But having said that, medical issues are very individual and the FAA's AME guidance suggests that one year is enough in some cases. In the end the only way to get a definite answer is to find a good AME who can review your case in detail and advise you. Note that standard advice is to schedule a consultation first, rather than go straight for the actual flight medical.
Disclaimer: I'm not a doctor or AME and I have no idea how brains work :-)

Thank you for this! Looking at that FAA page you linked, it would seem that I could be considered "favorably" and have to wait only one year.

@Deusdies I hope so, but please make sure that you get a consultation first. If you get an actual medical test then it can only end in a pass or fail, and a fail will complicate all your future tests. It's much better to get everything lined up properly with your AME and then do it.

If you are in the United States and you have never been denied an FAA medical certificate or had your medical revoked and you have a driver's license, then you can fly as a sport pilot in light sport aircraft.  There are some limitations compared to a PPL, the major ones being that you are limited to one passenger and daytime VFR operations.
If you decide to later move up to a PPL, then you will have to get a medical.  There is some risk, though.  If you attempt to get a medical and are denied, you will also lose your sport pilot priviliges.

Keep in mind the drivers license lets you self-certify yourself as medically able to fly (it is not a blanket pass to fly). If you are aware of an issue that would preclude your ability to obtain an FAA medical or otherwise be unsafe to fly, you cannot self-certify regardless if you haven't applied and failed a medical exam. You are held to the same standards you just have the ability to be the certifier rather than an AME. Advising someone to be a sport pilot to avoid taking and failing a 3rd class exam is bad advice.

My reading of Part 61 is that the light-sport medical requirements are a little more strict than this. Specifically, if you have ever applied for a medical certificate, then you need to be issued a medical certificate before you meet the medical requirements for light-sport aircraft. This means that if you're flying LSAs using a driver's license to meet the medical requirement, and you apply for a medical certificate, the act of applying causes you to be ineligible until your application is accepted.

I have a PPL license and I was on the way to obtain a CPL license when I had a seizure.
After many tests it came out that I had a cavernous angioma in my brain on the left temporal lobe anterior. It had bleed outside causing the seizure.
After removal of the cavernoma with brain incision, I had no other seizures and no complications and I went to see the AME.
I had lots of EEGs, MRIs and the AMEs decision is giving back my Class I certificate for at least 1 year after the surgery.
Neurology is decreasing the medication now and after medication is finished I will have few other checks.
If everything goes OK, I will have my Class I medical license back with an OML restriction (most probably).
This may give you lots of information about your situation : https://www.icao.int/publications/Documents/8984_cons_en.pdf
I hope this helps and don't give up. :)
Note : I have my Class 1 with OML and CPL(A) now.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Does the A380 have more than one auto-brake disengage switch?
I know that the A380 has low/med/high RTO auto-brakes. What I'm not sure about is if there's a switch near the thrust levers that can disengage the auto-brakes, other than the switches located on the brake panel.
To put it another way, the A380 has a brake panel where the switches allow you to engage/disengage auto-brakes (either low, med, or high RTO). But in addition to those, is there any other switch that can do the same thing without the pilot needing to take his eyes away from the view outside?

Well, he can tap the breaks.

@Hugh           yes he can, but it is far more convenient and safe to do it using a button - simply disengages the brake (which already slowed the a/c down significantly, below 50 knt)

Tapping the brakes is one. Increasing thrust past CLB (for go-around) is another. And does anybody know whether the brakes normally remain on until the aircraft stops or just until it slows down to taxi speed? I'd expect the later (in which case the autobrakes simply disengage themselves in the usual case), but I don't really know.

If the pilot disarms the Spoilers it will disconnect the auto-brake. Alternatively, the pilot may disengage the auto brake by setting the AUTO BRK Mode selector to DISARM.

@JanHudec I only have experience with the A319/320, and on those, the autobrake operates until disengaged or the aircraft is stopped. Proper procedure is to take over breaking manually before reaching taxi speed. Although I assume the A380 is the same, I cannot say for sure that it operates the same way.

@hagubear You can try to find it here.

The A380 has one Takeoff Autobrake setting which is RTO and five Landing Autobrake settings which are BTV, LO, 2, 3 and HI. Some may read that as four plus the BTV, which is a variable deceleration compared to fixed deceleration. As you have noted, these are selected from the Autobrake selector switch.
It is possible to disarm the Autobrake using a number of actions. One or two which have been mentioned already.

The Autobrake selector switch is set to Disarm.
The Brake pedals are depressed with a higher input than the Autobrake setting.
The Spoilers are retracted.
The Autothrust instinctive disconnect pushbutton switch is pressed. These are located at the ends of the Throttle control levers.
The RTO Arm pushbutton switch is set.

FCOM Reference for anyone interested - FCOM/DSC/32/10/30/20/00004244.0002001

Hello Kamran, welcome to Aviation.SE! Your answer is good as it is, but it could be even better if you would have the FCOM reference for it.

@DeltaLima Thanks. I am an engineer so I am unaware of the FCOM reference. The ATA reference for anyone interested is 32-42-00 (Normal Braking).

Having an extra engineer here is even better! Thanks for the reference. DL

@DeltaLima Thanks for the welcome! Will try and get hold of the FCOM reference after the weekend for anyone interested.

I found the following reference for anyone interested - FCOM/DSC/32/10/30/20/00004244.0002001

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What are the consequences of an airline pilot deviating from a flight plan to perform a random maneuver?
What are the consequences of a pilot flying a scheduled airline flight with passengers deviating from a flight plan to perform a random maneuver?
Say during a normal landing procedure, he spots something interesting not in his normal flight plan nor in the normal landing procedure, and decides to do a fly over, then returns to land the plane normally.
I imagine he would be fired from his company, but would there be legal action against him? If so, what would be the severity of the crime? (We can assume the US or Europe).

Gee, the last sentence is kind of an invitation for discussion and a little too broad.  Obviously he could hit him with a hammer or shoot him with a gun.

I can make that a separate question

https://aviation.stackexchange.com/q/66578/1796

Can you clarify exactly what scenario you mean? There's a big difference between someone ferrying a C172 solo under VFR to an uncontrolled airport, and someone flying a scheduled airline flight with another pilot and several hundred passengers on board. But both are commercial flights.

@Pondlife Updated

Something surprisingly similar to what you are describing actually happened, they collided with another aircraft, killing all on board.

As long as the pilots clear it with ATC and it doesn't violate the general rule against careless or reckless actions, there is nothing illegal about it. Pilots frequently request minor route changes, and ATC generally grants them if conditions permit. For instance, aircraft departing SFO often request a scenic pass by the Golden Gate Bridge as a treat for themselves and/or their passengers.
As long as no laws are broken (see above), their employer won't know whether the deviation was initiated by ATC or by the pilot, nor would they probably care even if they did know.
OTOH, if a pilot deviates from ATC instructions, it could potentially get them killed, so few pilots would knowingly do that. Deviations are almost always unintentional, and unless the pilot has a history of such problems that calls his qualifications into question, he/she will most likely get a "counseling" session and perhaps some refresher training.

"their employer won't know" That's not quite accurate. Almost all airlines are participating in a FOQA (Flight Operations Quality Assurance) programme. The airline might not directly get the data but if a particular pilot regularly requests deviations, this might come up.

@Bianfable FOQA is about safety and efficiency, and as long as key metrics are met, no alarm will be generated and no human will ever see the data. And unless a human reviews the radio tapes, they would have no idea who initiated any particular deviation out of dozens per flight times thousands of flights per day, nor any reason to care. And since most deviations are shortcuts (saving the employer money and passengers time), if anything, they want pilots to request deviations!


text:
Can a non-A&P fabricate and install his own hydraulic lines?
As a non-A&P, am I allowed to fabricate and install hydraulic fluid lines as part of preventive maintenance on an aircraft?

Is your aircraft type-certified or experimental?

Let us continue this discussion in chat.

Generally speaking someone who does not hold a mechanic's certificate may not fabricate their own hardlines or hoses, nor may they replace hardlines. This work must be done by someone holding the appropriate Airframe and/or Powerplant mechanic rating.
An owner/operator with a private pilot certificate may install prefabricated flexible fluid carrying hoses (with the specific exception of hydraulic hoses) as preventive maintenance,as allowed under FAR 43 Appendix A.

There are three notable exceptions to the above general statements:

An owner of a Special Light Sport aircraft may acquire a repairman certificate, enabling them to perform this kind of work on their aircraft.
The builder of an Experimental Amateur-Built aircraft ("homebuilt") may perform this work on aircraft they have built, as FAR 43 does not govern maintenance on aircraft with an Experimental airworthiness certificate.
If working under the supervision of an A&P mechanic you are authorized to perform any work your supervisor is authorized for (per FAR 43.3 (d), and subject to some specific requirements).

Wouldn't an owner be able to fabricate them as owner-produced parts?

@Pondlife That's true - there is a 4th exception in that you could go the owner-produced parts route for pretty much anything if you own the aircraft, have the resources to properly make the part, and your mechanic considers it airworthy & will install it. (Hydraulic lines might be stretch for this depending on the system's rated working pressure, though I suppose if you custom-order hoses from Aeroquip or someone they're technically "owner-produced" because you're providing the specifications).


text:
Why do model aircraft fly and maneuver so differently from real aircraft?
I recently viewed on Youtube many videos of small and giant models of airplanes. In some cases they are small (for example an A330, 1 meter long) and in others they are huge (for example an A380, 5 meters long).
As far as I can see, the smallest ones seem to have electric engines (maybe fake jet engines in which fans act as propellers?) but the bigger ones seem to have internal combustion engines, even if I don't know how they run.
Anyway, the general impression is that even from a great distance it's obvious that they are models and not real aircraft. They are all too quick, too agile, make very short takeoffs, have very low inertia, make quick turns and so on.
Can you tell me why these handling differences exist in models that otherwise tend to simulate with great precision every other detail of the real aircraft (lights, shape, colors, fine details, landing gear, etc.)?

Very relevant discussion on Space.SE: Can a miniature Saturn V get to the moon and back?

tl,dr; Cube-Square law in effect.

"Dynamic similitude" explains why small aircraft move more rapidly than larger scale ones. For RC and real comparison there are other factors too (real aircraft have stability augmentation systems and several layers of protection against over stress, etc). Good observation you made there.

Related question

Ask Reynold, if you have his number.

Another factor:  If you're going to scale everything you should also scale your camera.  Watch your model in slow motion and it will be more realistic.

^ If you watch model aircraft videos in slow motion they definitely tend to look more like the real thing. Also for real aircraft that kinda look like the way models fly check out some ultralight/stunt plane videos (and be amazed!)

Make a housefly the size of a horse and see if it can still walk stuck to the ceiling.  Make a bumblebee the size of a vulture and see if it can still fly.  Length, surface area, and volume change at different rates as linear size grows or shrinks.

You all seem to forget how time scales: Inversely with the square root of size. For a quarter scale aircraft all manoeuvres happen at twice the speed.

@PeterKämpf Look three comments above yours.

@PeterKämpf This particular scaling of "time" is a derived quantity - it's a consequence, or result, of more fundamental scaling laws.  If you really want to explain why this behaviour occurs you have to start deeper.  There are other physical systems where "time" does not scale this way.  Dimensional analysis is required to come to this conclusion.

It's partly inherent in the way things scale.
If you double the length of the model, then the wing area (length times width) increases by a factor of 4, but the weight and volume (length times width times height) will increase by a factor of 8 ... so doubling the size means halving the weight-to-lift ratio.
In the most extreme cases, a tiny model will blow away on a puff of wind, and a huge model (bigger than the real plane) can't take off at all.
I suppose you might theoretically try to make small models harder to fly, by adding extra weight.

The above is theoretically true but maybe nonsense in practice: it assumes that structural materials become thinner when the model is scaled, in reality the structure isn't even the same material.
So let's look at it another way:

A full-scale A380 weighs let's say 500 tons, length about 70 metres.
Decrease that to 1 metre model and the surface area has decreased by (70x70=) 5000.
So for the model to have the same weight-to-area as the full-scale plane, it would need to weigh (500 tons / 5000 =) 100 kg.

Your 1 metre model presumably weighs much less than 100 kg, therefore it has much less weight-to-area ratio. QED.

It's also important to consider the Reynolds number, which depends on the air's viscosity and density, and on the size and speed of the model. The Reynolds number affects turbulence, which is very important to a wing's lift (for an example of how even a tiny change has a large effect, see Can a sandpaper-thick layer of ice reduce lift by 30 percent and increase drag up to 40 percent?).
To get the right Reynolds number for a small model you must increase the density (e.g. pressure) of the air, or increase its speed. But given the ordinary speed of aircraft, you couldn't increase (scale up) the air speed because it would become super-sonic, which would change the scenario.
Based on this answer to 'Understanding the Reynolds-number scaling problem', and the comments below it, I think that a 1-metre model of a 70-metre A380 (so a scale of 70:1) might behave like the full-scale model if it were flown under the following conditions:

air density is scaled up, so 70 atmospheres of air pressure
lift and drag are scaled down, so:

weight of the model is 7 tons (instead of 500 tons)
thrust of the model is 4,000 lb (instead of 300,000 lbs), i.e. about 2 tons

air speed is realistic (e.g. 150 knots to take off)

Obviously this would be quite unusual for a model airplane1.
1Air liquifies at 60 atmospheres; and the model would need a specific density of about 100, i.e. 5 times heavier than gold or uranium).

I am wondering ... wouldn't it be necessary to flight "real" airspeeds if one would make model with increased weight and real wing loading?

Actually the Reynolds Number is the key to this question and it affects every body moving in a fluid, including boats in water (Hydrodynamics) as well as aircraft in the air (Aerodynamics).

Model aircraft are generally built with much lower wing loading and much higher power to weight ratios. This can be done partially because they have no real payload, and do not have to fly for long durations.
Having a plane that is more lightly loaded and has more power results in the traits you mention. Also, being lighter means wind gusts and other turbulence will have a greater impact and result in more rapid directional changes.
I should also note that the extra power and lower wing loading is desirable in a model because a remote pilot does not have the same instrumentation and physical sensory input that is gained by being in the plane and helps to safely fly closer to the limits of the aircraft.

The laws of physics are not scale invariant.
Area scales with the square of dimension while volume scales with the cube of dimension. Aerodynamic effects roughly scale with area. Mass roughly scales with volume. Inertial scales with mass. Moment of inertia scales with mass times dimension.
The end result of this is that the models have far stronger aerodynamic effects compared to their inertia. This makes them far more nimble than their real-world counterparts. OTOH the real-world aircraft are generally able to fly faster and further and have a better fuel consumption per ton-mile.

Not to mention that material strength usually roughly depends on area as well, so you can use much lighter supports that are needed in the full-scale plane. Most model airplanes are much lighter than a down-scaled plane would be, further reducing inertia and increasing lift.

You might want to add that time scales inversely with the square root of linear scale. A quarter-scale aircraft performs its manoeuvres at twice the speed.

I'm not sure whether you are asking only about the physics involved, so hopefully this isn't too tangential - but one factor that's not really been touched on much is pilot input.
In addition to dealing with the disruptive influences that people have already mentioned, the pilot of a model would have to ignore its actual maneouvering capability and use a lot of restraint - very, very small and precise control inputs, unneccesarily slow acceleration, and so on - to achieve convincing scale flight behaviour.
With that approach (and in super-calm conditions) I think you'd be surprised at what can be achieved with large models - you've probably seen radio-controlled model work in movies without realising it. However outside of applications like that, there must be a strong tempation just to wang the model around, because flying is fun!
Of course you're unlikely to see much model flying in films now, since flying machines are one of the things that it's quite easy to render convincingly using CGI. But for a bit of historical context, there's a small but interesting gallery here featuring some of the model aircraft from the movie The Battle of Britain, which was noted for its outstanding model work (considering it was made in the sixties).
http://www.daveswarbirds.com/bob/models.htm

Interesting alternative point of view. Thanks Mike! :-)

It depends on the pilot, too. I have seen a guy fly a 5-foot wingspan model of  Piper Cub just as if it were the real thing. Much longer than necessary (for the model) takeoff run with lifted tail, held back on the throttle to simulate scale speed, flew the pattern for the landing, and rolled it out. Very pretty flight. But he could have hot-dogged it like the OP's description.

Excellent point -- being able to maneuver quickly doesn't mean that the model has to maneuver like that.  The physics & aerodynamics described in the other answers explain what the model is able to do, but the pilot is still, presumably, in control of what it actually does.  Nice answer, and welcome to aviation.se!

See this very relevant discussion on Space.SE: Can a miniature Saturn V get to the moon and back?
The  square-cube scaling problem was already mentioned in other answers, but another important factor will be that the Reynolds number of the air does not scale with the model. You can think of this as the air being more viscous from the smaller scaled craft's point of view, which increases drag without providing any additional lift (Thanks to Vladimir F in the comments for corrections).

First, "Reynolds number of the air" is a very strange formulation. There is no such thing as Reynolds number of the air. There is a Reynolds number which describes some physical  problem, but it is not a property of the fluid, it is a scaling parameter of some configuration!

Second, it is quite wrong to describe the effect of smaller Reynolds number as making the air thicker. Larger kinematic viscosity maybe, but not higher density! Perhaps higher density would help flying by increasing the lift and the drag at the same time, but increasing viscosity just increases the drag. And indeed at lower Reynolds numbers we will see much higher drag. I tried to explain this confusion already in http://aviation.stackexchange.com/a/21156/3189  The xkcd comic is not really relevant here. Even worse, it is misleading to point to it.

@VladimirF: Thank you for your clarifications. Note that the OP is clearly not an AE, therefore I submit that in context, discussing kinematic viscosity would be no more lucid than saying simply "thicker".

My point with density-viscosity is because your link to XKCD which is highly misleading. The XKCD comic is about density. Higher density does help flying. That's why there is service ceiling. You cannot fly too high because the density of air is too low at certain height. However, viscosity (and Reynolds number is all about viscosity) does not help anything, especially at low Reynolds numbers. It just causes drag. (let's not complicate things with Kutta condition and viscosity)

You're right, and I'm going to edit my answer. The scaled craft will see increased drag, but no increase in lift. I was completely thinking backwards. Thank you.

Most of the precedent answers are right, I'm just trying to put this in laymen's words.
The construction of an airplane requires fine tuning of shape, material composition, volumes, masses, areas of various elements such as body, wings, control surfaces, engines etc.
Now think of all these elements being optimized for the real scale plane, so that it can fly perfectly in the aerodynamical conditons it is designed for, namely airspeed (which involves distance), air density (which involves volume), lift, drag (which involves area) and weight (which involves mass). All this is the result of aeronautical engineering and the equations of fluid mechanics.
Now, as pointed out previously, when you scale distance, then areas, volumes and masses scale differently.
Notably area goes with distance^2, volume with distance^3. Mass goes roughly with volume, but depends on what materials the model plane will be made of, which are likely different than those the real plane is made of.
So it becomes obvious that the reduced model plane operates in completely different aerodynamical conditions than the real plane. Hence the radically different handling characteristics.

Don't overlook how time scales: Inversely with the square root of size. A quarter scale aircraft will "look" as if time "flies" at twice its normal speed.

@PeterKämpf: I know what you mean. The problem is that time should scale in order to accommodate for the change of scales of the other dimensions and still fulfill the same fluid dynamic equations. But it doesnt. Time is the same for model aircraft and the real. Thus even though the model plane certainly flies at much lower absolute airspeed than the real plane, it looks like it is flying too fast compared to its size.

hoehne and Swapnil already noted that when the model is capable of the outstanding maneouvres doesn't mean the pilot must perform them.
Models, whatever they ale model of, are more agile thanks to higher power to weight ratio and more agile engines. Delays between idling and full throttle for real craft are much longer than delays for model engines.
Another difference is in volume(mass)-area ratio, it is not constant. This allows the model plane to fly with slower velocities.
Also the flaps and rudder effective area is different for model and real craft. And their deflection angles are higher for model planes than for real ones.
To build a fyable model you must alter the position of the centre of mass. If you scale everything down, the plane won't be able to fly at all.
Modelling, in the scientific way, is real and nontrivial problem and one must be super-cautious to extrapolate (downscaled) model data to real (upscaled) problems. And hydrodynamics and aerodynamics are the most difficult ones of all.

The scaled model is flying at its full potential but whereas the actual one is  following global airline standards for safety and flying and manoeuvring in its safety zones.
We don't want our airline pilot to take sharp turns just because the plane could , Do we?

authors: Vladimir F Героям слава


text:
How should I handle a change of address for a Student Pilot License in the USA?
I have moved to another place from current address with in same city, Do I have to change my address to the new one on my FAA Student Pilot License? If yes, how do I do it?

You can change your address with the FAA online.  You do not need to get a new plastic certificate.
http://www.faa.gov/licenses_certificates/airmen_certification/airmen_services/

Thanks, i found out that for the student pilots also has to update their student pilot document


text:
To become a BGI, what are the minimal requirements from the FAA?
To become a Basic Ground Instructor (BGI), what are the minimal requirements from the FAA?
I am especially interested in understanding the minimal certifications (PPL/CPL) that are needed to obtain the BGI.

Related? What is the step-by-step process to become a ground instructor in the US?

What is BGI? I've been flying since 1994, never saw that acronym.

@CrossRoads - "Basic Ground Instructor"

Ah - I skipped Ground instruction, just studied on my own and anything I couldn't figure out for the Knowledge exam (both Private and Instrument) I worked out with my flight instructor.  Ground school with a bunch of non-engineers had no appeal to me.

The answer to your question is found in FAR 61.213
For your convenience I've copied and pasted the entire thing here, but you should reference the official link as regulations change from time to time. Highlighting is mine.
Note that no pilot's certificate of any kind is necessary.

(a) To be eligible for a ground instructor certificate or rating a
  person must:
(1) Be at least 18 years of age;
(2) Be able to read,
  write, speak, and understand the English language. If the applicant is
  unable to meet one of these requirements due to medical reasons, then
  the Administrator may place such operating limitations on that
  applicant's ground instructor certificate as are necessary;
(3) Except
  as provided in paragraph (b) of this section, pass a knowledge test on
  the fundamentals of instructing to include-- (i) The learning process;
  (ii) Elements of effective teaching; (iii) Student evaluation and
  testing; (iv) Course development; (v) Lesson planning; and (vi)
  Classroom training techniques.
(4) Pass a knowledge test on the
  aeronautical knowledge areas in-- [(i) For a basic ground instructor
  rating, Secs. 61.97, 61.105, and 61.309; (ii) For an advanced ground
  instructor rating, Secs. 61.97, 61.105, 61.125, 61.155, and 61.309;
  and] (iii) For an instrument ground instructor rating, Sec. 61.65. (b)
The knowledge test specified in paragraph (a)(3) of this section is
  not required if the applicant:
(1) Holds a ground instructor
  certificate or flight instructor certificate issued under this part;
(2) Holds a teacher's certificate issued by a State, county, city, or municipality that authorizes the person to teach at an
  educational level of the 7th grade or higher; or
(3) Is employed as a teacher at an accredited college or university.

Because the OP specifically asked about it, you might like to mention that there's no requirement to hold a pilot's certificate of any kind

@Pondlife Good call!


text:
Why is it safe flying in a tailwind when taking off is not?
If taking off in q tailwind is unsafe (due to loss of lift from reduced air speed), then why is it safe flying in tailwind, such as in jet stream?

Does this answer your question? Why is tailwind during final approach and landing so dangerous? and see also: How do jet streams affect the fuel consumption?

Do any of these questions help? Here, here, here

Cruising speeds are also generally quite a lot higher than landing / take-off speeds - so the effective loss of lift is not relevant - 100% - 2% is still 98%... At landing speed 100% - 40% - well you get the idea ....

BTW, I downvoted because the statement concerning loss of lift from reduced airspeed is not clear.  Yes, you will have reduced lift at lower airspeeds, but with a tailwind you will simply take off at a higher groundspeed further down the runway.  Hint:  Airplanes don't rotate after a fixed ground roll distance, they rotate at a predetermined airspeed.  Always...

Another hint:  If you are flying in the jet stream at 300 knots of indicated airspeed you will have the same IAS no matter whether you have a tailwind or a headwind.  The ONLY thing that changes based on direction of flight is ground speed.

Why was this downvoted?

I once had a flight instructor tell me, "As soon as you get airborne, there's no wind." While this is not strictly true in all cases (wind shear is still a thing), it does hold true for flying with the wind. The aircraft is moving in the moving air mass, so the fact that the wind is "coming from" behind you only helps you get to your destination faster. Aircraft can fly at the same airspeed no matter which direction the air mass is moving.

Or how fast the air mass is moving.

What about a tailwind matching the max speed IAS an aircraft can gain, and thus reducing IAS to 0, with no the wings generating no lift?

With tailwind you fly at same IAS but increase ground speed. You look at IAS and ignore ground speed.

@HarshilSharma In that case, the IAS of the airplane wouldn't change, but its ground speed would double.

“Wind” is simply an air mass moving over the ground. When flying, planes do not experience wind per se because they are flying at a (mostly) fixed speed relative to that air mass.
We speak of a “headwind” or “tailwind”because the movement of the air either reduces or increases the plane’s speed relative to the ground.
We want to takeoff and land with a headwind because the reduced ground speed means we need less runway to achieve the same speed relative to the air, whereas a tailwind means we need more runway.

From what I think, more important than runway length required, a tailwind will require more engine power to maintain sufficient airspeed to prevent a stall. Which is what makes me think that a jetstream can theoretically reduce an aircraft's airspeed and may remove all lift. It's not a problem if jetstream is at 300 kph and the aircraft is moving at 800 kph IAS. Can, theoretically, would a 800 kph tailwind possibly stall the aircraft?

@HarshilSharma sudden increase of tailwind can stall an aircraft, see e.g. Delta Air Lines flight 191. The jet stream is, however, hundreds of kilometers wide and kilometers high, so even though the wind speed can be high, the change when entering or leaving it is gradual, so the aircraft will catch up it easily, the airspeed remaining constant and just change in ground speed indicating the presence of a jet stream.

@HarshilSharma Two side notes: 1. transport aircraft never move at 800 km/h indicated. 800 km/h true airspeed is rather low, but due to the low air density up in the cruise flight level the indicated airspeed is only around 250 knots (460 km/h). 2. in cruise with flaps retracted the margin to stall isn't that large, so a 300 km/h wind shear would almost certainly stall the aircraft—fortunately the jet stream is never sudden.

@HarshilSharma, no, tail wind does not require more engine power to maintain sufficient airspeed. The aircraft flies relative to the air, so the power to airspeed is always the same. The only problem with tail-wind is that the runway is moving aft under the air mass while the aircraft is taking off or landing, so more of it is needed.

@HarshilSharma Incorrect. I normally cruise at 75% power, which results in an airspeed of 110kt. If there is a 10kt tailwind, that means a groundspeed of 120kt. If there is a 10kt headwind, that means a groundspeed of 100kt. My plane stalls at an airspeed of about 45kt; my ground speed is irrelevant.

If taking off in q tailwind is unsafe (due to loss of lift from reduced air speed)

This is incorrect.  The airspeed required to take off is the same, no matter the speed of the wind relative to the ground.
Taking off in a tailwind is potentially unsafe because the required ground speed is higher.  If you take off in a tailwind, you may run out of runway before your airspeed is high enough.
You cannot run out of runway while in flight.

Why was this downvoted?!

@MichaelHall -- Maybe someone thought the quote was correct-- in the sense that at any given point in the take-off run (specified either in terms of elapsed distance, or elapsed time), the airspeed will be less when there is a tailwind.  I'd say the answer would be improved by deleting the very first sentence after the quote.

But the airspeed will NOT be less, the ground speed will just be greater.  (Unless you say airspeed compared to ____.)  This will affect time to accelerate, ground roll distance, tires speed, etc.

The word "less" always implies a comparison, so I stand by my comment.  Of course, the airspeed at the moment of liftoff will not be less, normally.  The quoted statement  is vague enough that you one can argue either way, that it is correct or not correct.

No, it doesn't imply any comparison!  If you rotate at 80 knots you should ALWAYS rotate at 80 knots whether you have a headwind, tailwind, or no wind.  Period.  Therefore airspeed will NOT be less - instead ground roll, ground speed, and time to accelerate will all be increased with a tailwind.  This is the fundamental misconception that non-flyers can't seem go get over, and it baffles me because it really isn't complicated...

OK, so to clarify, you absolutely cannot simply say that "airspeed will be less" without a modifier.  Because with a tailwind the statement "airspeed will be less after 1000' of ground roll" is true, but "airspeed will be less at rotation" is obviously not.


text:
Why is wing anti-ice in transport aircraft not required below -40 degrees Celsius?
Okay, this confuses me. As it gets colder doesn't it mean more possibility of icing? And is the -40 degrees SAT or TAT?

I'm curious: where do you get the -40°C limitation from? The anti-ice systems description for the §25 aircraft that I am most familiar with states that anti ice should be used between OAT of +10°C and -30°C. However, the aircraft limitations require that all anti-ice systems must be on when operating in visible moisture at an indicated RAT of +10°C or lower.

@JonathanWalters it's not specifically mentioned in my company 737 limitations. I get that information from 737 limitations of the other company which operate in areas known for icing condition

That -40 is SAT and in degrees Celsius.
The reason that anti-ice is generally no longer needed below that temperature is because at that point, the air is so dry that there isn't enough moisture left to form frost or ice that will stick to the airframe or engine parts.
Normally, it's engine anti-ice, rather than wing anti-ice, that is still on when you get to the point in the climb that the temperature drops below -40. At least in the 737, the engine anti-ice is kept on continuously while the temperature is below +10 TAT and above -40 SAT and you're in IMC or other visible moisture (i.e. rain, snow, etc), while wing anti-ice is only used "as needed" to remove ice buildups from the wings.
Also of note, the "below -40 is too cold" logic only exists in climb and cruise, not in a descent. The rationale there being, you don't want to miss the transition from the air is no longer "too cold for icing" as you descend, and find out that your motors ARE now accumulating ice, when they start to act sick... that makes your day entirely too interesting!

Celcius > Celsius

Note that the -40 is also in degrees Fahrenheit :)

Well, dry in what sense? There can be still clouds in that air, but they are in the form of ice already. There is no supercooled water under -40.

I think "snow" behaves similiar, if it becomes very cold the air will contain less moisture and snow-flakes become very tiny.

There is some serious confusion around. If you cool down an air parcel then the relative humidity rises! You can easily have 100% relative humidity and -40°C. The absolute humidity will be probably small, because the after the saturation the superficial water will condense and freeze and form snow. But just saying that cold air = dry air is really incomplete.

@IanF1 I am a little bit puzzled, does that mean colder than -40 degrees fahrenheit temperature is no icing condition or something else?

@AziziAkbar -40 F and -40 C are very close, so saying "below -40 C" is nearly equivalent to saying "below -40 F".  Warmer than that, the F number is larger (ie at the freezing point of water, the F number is 32 while the C value is 0), while colder than that, the reverse is true.  SAT and TAT are always reported in Celsius, though.

@RalphJ whoa, that's where I missed. I tricked by thinking that there is no convergence in numerical value between celcius and fahrenheit when stating the temperature. Thank you good sir!

@VladimirF so does that mean the ice will no longer stick to the wing/engine-icing-susceptible-surface in such condition?

I think thi important fact is the absence of supercooled water which sticks really fast. Snow does not, it is blown away. The water vapour content is small indeed and the icing from that can be neglected. Even at sever icing conditions it is mainly the supercooled water that sticks and freezes so quickly.

@RalphJ: More than that - they are exactly equivalent.

authors: Vladimir F Героям слава


text:
How do we derive Lift Coefficient of an airplane's wing?
In many references I read that lift coefficient ($C_l$) is a product of wind tunnel test. But many times I read the formula to calculate the coefficient of lift. From the lift equation, $$L=\frac{1}{2} \rho v^2 S C_l$$ $C_l$ is rearranged to derived the $C_l$ equation as below: $$C_l=\frac{2 L}{\rho v^2 S}$$
In my opinion, even it correct mathematically, but is not correct in the engineering process as it is not a product of a mathematic. The Coefficient of lift ($C_l$) is required to calculate the lift that will be produced by an airplane's wing with the given speed and the wings' area.
Am I correct with my opinion? Or, is any formula to calculate the coefficient of lift so the lift force can be calculated without any wind tunnel test?

The lift coefficient accounts for complex aerodynamic effects that are impossible to calculate by hand in any practical way. I'm not sure what you mean by "In my opinion, even it correct mathematically, but is not correct in the engineering as the it is not product of mathematic." My best guess is that you're pointing out that even though you can rearrange the lift equation to calculate $C_L$, it's not derived from first principles, which is correct. It's a convenient way to represent more complex interactions that are difficult to calculate.
Essentially, we know lift varies linearly with dynamic pressure ($\frac{1}{2} \rho V^2$) and it varies linearly with surface area. So the lift equation may be written, in its most reduced form,
$$L = C_L q S.$$
Notice the similarity to the drag equation:
$$D = C_D q S$$
And the pitching moment equation:
$$M = C_M q S c$$
Each of these quantities varies linearly with dynamic pressure and surface area, so it's useful to express them as products of these quantities. Changing either of these variables will probably affect the aerodynamic coefficient as well, but for modeling purposes that don't require extreme precision, in most cases, the aerodynamic coefficient won't change nearly as much as dynamic pressure can.
As a result, knowing $C_L$ and $C_D$ for a variety of flight conditions, you can model your body of interest with fairly good accuracy without having to go through the complexity of computational fluid dynamics, which is the numerical way to predict aerodynamic forces.
To answer your question about a formula to calculate the coefficient of lift, there are methods that will allow you to get a rough estimate (e.g. lifting line theory), but to get results comparable to wind tunnel testing, you have to use computational fluid dynamics, which will require a computer to achieve meaningful results.

Apologize for the absence of some words made the passage not easily understandable.

My point is whether is correct to write coefficient of lift ($C_l$) as $$C_l=\frac{2 L}{\rho v^2 S}$$ as the $C_l$ is the thing we have to find so we can calculate the lift ($L$). That $C_l$ is derived from tunnel test or from computer modeling (as you mentioned). But for sure, not from calculation which is involve $L$ that we don't know. That will be like the question, which one is first, chicken or egg.

@AirCraftLover sorry it took me a while to get back to this. You can certainly write the equation that way to solve for $C_L$ if you're taking measurements of the lift (i.e. you know $L$, probably in a wind tunnel) but if both $C_L$ and $L$ are unknown, you can't solve for both in a single equation. In that case, you would likely have to use CFD.

And should be the $C_L$ is firstly derived before completing the aircraft design, right? That actually my concern as many explaining the mathematical formula which it is seem correct mathematically but wrong in the actual implementation as we have to derive at first hand the $C_L$ before continue to rest.

@AirCraftLover It depends on what you're trying to accomplish. If you have an idea for an aircraft and nothing more, then no, you can't use that formula to find $C_L$. You can get a rough estimate by taking the section lift coefficient $C_l$ of the airfoil and reducing it slightly to account for lift loss from wingtip vortices, but that doesn't take into account contributions or losses from the fuselage, horizontal tail, etc. Before completing an aircraft design, you'd want to have built multiple models and tested them in a wind tunnel, where you could find $C_L$.

It's important to stress that $C_L$ is by no means a driving factor in aircraft design. You're generally more concerned with range or endurance and the lift to drag ratio, which $C_L$ can be helpful in determining.


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What is the Efficiency of Cessna-150's Propeller?
From that Cessna-150's data, it used Continental 0-200-A 100 HP (75 kW) at 2,750 RPM and with Fixed Pitch propeller with its diameter=69 inches. If power is delivered maximum to the propeller (through direct shaft), then what is power of the propeller to push (thrust) the airplane?
Engine (100HP) ==> Propeller ==> Thrust (? %)
Additional Data of Cessna-150:
Weight of loaded: 680 kgs.
Weight of Empty: 437 kgs.
Wing loading: 9.4 psf (45.8 kg/m²)
Power loading: 15 lb/hp (6.81 kg/hp)
Maximum Engine Power: 100HP (75kW).
Wing Span: 10.2 m.
more detail about the plane can be seen here: http://www.skytamer.com/Cessna_150C.html

It varies widely depending on the propeller pitch, diameter and forward speed at the time but the rule of thumb I use is around 3-4lbs of thrust per HP while stationary.  Lycoming O-320s with fixed pitch props make a static thrust of somewhere around 500 lbs.  More with a constant speed prop.  An O-200 makes a little over 300 lbs static.

Actually what I am asking is specific to the Cessna 150. I added another info my post to give clearer information. Power loading is said 15lb/hp (or 6.81kg/hp) and the propeller 's diameter is 69 inch. Indeed no data of the propeller's pitch. And it is actually part of my question here. As said in the info that the pitch is fixed, that actually my question what is the pitch and how many the thrust power delivered by the propeller. Do you have please reference how to calculate its mathematical?

Probably quite low... After a series of gliding experiments made with a Luscombe 8E, and published by the AIAA, the conclusion was that the prop efficiency was around 62%...
https://engineering.purdue.edu/~andrisan/Courses/AAE490A_S2010/Buffer/AIAA-46372-872.pdf

Thank you Xavier. Interesting. I will read it thoroughly.


text:
What is the most effective of propeller's rotation in a two-propeller engine airplane?
Consider two-propeller airplane as picture below (picture 1 (the above) and picture 2 (the below)). Which one is the most effective configuration to produce thrust power, stability, structural design, etc if still any?

Picture 1 (above): the bigger angle of attack (due to parallel to the gravitation) is meet near the fuselage, in between the two propeller's axis.
Picture 2 (below): the bigger angle of attack is outside the two propeller's axis.

Need physic analysis to answer this question.

TL;DR: The first one.
It all comes down to P-factor.

Airplanes fly at a positive angle of attack, so as you can see, this AoA for the wing translates to positive AoA for the descending blade. Vice versa, the positive AoA for the wing is negative AoA for the ascending blade. I.e., the descending blade has more thrust than the ascending blade under AoA.
Now it is pretty obvious why you want the descending blade on the inboard and ascending blade on the outboard, because in a single-engine-out incident this creates a less yawing moment to the otherwise arrangement.
However, life does not always turn out as we want it, as the engineers at Lockheed learned the hard way when testing their XP-38. They had gone through the trouble of using handed engines and props, making the starboard engine turn counterclockwise and the port engine clockwise. Now the fighter appears to be perfect: no torque, no gyro, a minimum yawing moment in a single-engine-out situation. But the XP-38 had pitch control issues which were later found out to be caused by the downwash created by descending inboard blades hitting the wing section between the boom and the central nacelle, creating a disturbed airflow trapped between the twin booms and finally affecting the elevator. In the end, they had to switch the engines' positions so the P-38 ended up with two critical engines.

Can you give physic/mathematical calculation, please with both configuration? What is the aspect affected so the the first configuration is better than the second.

@AirCraftLover what calculation do you need? the inboard blade has more thrust so the overall yawing moment is smaller than otherwise

I still can not get the reason why the inboard descend is better than the other one. That the reason why ask mathematical or physic calculation to see what make them different between the first option and the second one. Apologize for my lack of my understanding.

@AirCraftLover let the inboard thrust be T, outboard t. let the hub of the prop be D away from airplane's central line. let the distance from the hub to the aero center of each blade be R. inboard descending configuration has yawing moment (D-R)T+(D+R)t, inboard ascending (D-R)t+(D+R)T. which is larger?

Give me some moment to understand.

Apologize my friend. What is the aero center of each blade, R?

@AirCraftLover R is the distance from the center of aerodynamic force acting on each blade to rotating axis of the propeller

Can you provide picture to describe it, please?


text:
Which one needs longer runway of an airplane between take off and landing for a constant weight?
Consider we have an airplane that the weight will be constant when take off and landing (in fact will be no such common airplane right now as the fuel will be less due to engine consumption. But in an electric airplane, that will be possible, I guess). What I am asking is, which one is required longer runnway from an airplane starts taking off in the edge of the runway until it just leaving the runway (ground), compared to when that airplane is landing, from it start touching the runway until it can stop safely in the runway. How do we calculate the required runway for landing?

Unclear what you're asking.  I very much doubt that an anvil would be at all helpful in takeoffs or landings: https://www.merriam-webster.com/dictionary/anvil

Which one is need longer runway of an airplane between take off and landing for an airplane with the same weight?

Apologize if it was not clear. I have changed it to runway. Hope it clear.

As far as my understanding, there is equation to bind Lift force (L) to the air density (rho), the airspeed hit the wing (v), the wing area wide (A), and the coefficient of lift of the wing of a specific airplane (Cl). L=0.5rhov^2ACl. The weight of course we can get. Lifting force is the opposed force to the gravity+drags. When the Lift is same with the gravity and drags, then it is the time airplane to leave the ground. That what my understanding. And the requires runway we may calculate based on the required speed and the acceleration. But so far I don't have such a formula for landing.

That supposed to be only simple thing. I just don't have such that formula that bind the known airplane data and the speed required to land, from it touch the runway until it stop (without a brake). And why I say constant weight, just to make the question more simple and avoid too many question. So what I am asking is in form of a formula, not in number.

That above formula should be **L= 0.5 * rho * v^2 * A * Cl. ** I have to use space to avoid it detected as formatting tool.

I don't think you can find an answer that's applicable to all aircraft.  Takeoff distance depends very strongly on engine power.  (As an extreme case, the space shuttle needs zero runway to take off, lots to land.)

Generally, aircraft require longer distance for taking off than landing roll.
The standard definition for take-off phase is from the start of acceleration (stopped aircraft) to the point where aircraft reaches 35 ft above ground. Similarly, the landing phase is from 35 ft to the final stop. What you mentioned is a sub-phase, which is called "ground roll".
When the aircraft is accelerating for the take-off, the engins' thrust must overcome the aerodynamic drag and the ground friction, and the resultant will contribute to the acceleration. But when the aircraft is landing the reversed thrust in addition to an increased aerodynamic drag (due to spoilers and increased flap deflection) and increases ground friction (due to application of brakes) are decelerating the aircraft. This is physical reason.
While there are some engineering methods (such as equations or charts, usually presented in aircraft design books i.e. Roskam or in aircraft performance books), the most accurate method is to writing the force balance acting on the aircraft and computing the acceleration at any time step, and solving the position differential equation for the distance. But this method requires details about aerodynamic and thrust characteristics. This method somewhere was called time marching.

Don't you have something like formula how to calculate or to estimate the runway requirement for landing? Appreciated if you have.

Landing distance will always be shorter, due to higher deceleration values but mainly due to the definition of TO length, which takes into account obstacle clearance and rejected TO distance after engine failure.
Torenbeek, Synthesis Of Subsonic Airplane Design, section 5.4.6 gives the runway length from touchdown to standstill as:
$$S_{run} = \frac{{V_{td}}^2}{2 a} + V_{td} \cdot \Delta t$$
With

$a$ = max deceleration.
$V_{td} \cdot \Delta t$ = mean inertia time to reach max deceleration, approx. 1.5 to 2 seconds (from Torenbeek Appendix K)

Values for $a$/g on dry concrete are given as:

0.30 - 0.35 light aircraft, simple brakes
0.35 - 0.45 turboprop without reverse thrust
0.40 - 0.50 jets with ground spoilers, anti-skid devices, speed brakes.
0.50 - 0.60 as above with nose wheel braking.

For wet concrete, as in this answer


text:
How to calculate or design glide ratio?

Was interesting to watch the explanation about glider here, especially about the glide ratio. Glide ratio is defined as the ratio between altitude when it start gliding (Y-axis) to the distance it can reach until the altitude reach zero (X-axis), as shown in the picture 2. Gliding is a descending, opposite of climbing, which is ascending. All airplanes are designed to be able to glide to anticipate the worst case when the engine is failure.
There are many discussions how to calculate an airplane when ascending, explains the relation of wing area, wing ratio, airplane load, initial speed (velocity), and the power it requires to meet the ascending rate. Then my question is, how do I bind such that information when calculating the glide ratio/distance?
Edit:
I believe the weight of the airplane contributing the glide distance (ratio) it can reach. Just consider two identical airplanes (say it Cessna 172), one is fully loaded and another one is not loaded at all (except the pilot, of course). They fly at the same speed and turned off the engine at the same time.
Also consider two airplanes (also Cessna 172) with exactly same weight and load, but fly at different speed and then turned off the engine at the same time.
From both above illustrations, we know their momentums are different. Is not the momentum contributing to the glide distance (ratio)?

I believe the link suggested duplicate is different. If we just consider the LIFT and the DRAG ratio only, it will not be the answer. In my limited knowledge, there will be the initial velocity (Vo), the initial when an engine is off or when we start jump from a heigh if it is pure glider. Beside, I believe total weight also affecting the gliding ratio. A heavy weight however absolutely different from than light weight. We may check it from force components. So, we have to look back the answer rather than to take the answer in the provided link. So I think, better reopen this post.

One simple way is to compare thrust requirement for level flight at Vbg, where lift to drag ratio is optimal, to the weight of the plane.  This ratio of pounds of thrust, which is equal to pounds of drag in steady state level flight, to the weight of the plane should give a good approximation of the glide ratio.  Glide ratio is how much altitude do I give up to move how far forward.  The altitude is the power source.  How far forward you move is dependent on drag.
So an 1800 lb Cessna would require 200 lbs of "thrust" to glide 9:1.
Now to design for it, speed is a critical factor in selection of wing type and loading.  This is the albatross vs eagle consideration.  Higher speed cruisers favor the albatross.

As my comment above, to determine the gliding distance or we call it gliding ratio, the initial speed (velocity) and it weight absolutely affecting. We may imagine, a heavy loaded airplane will have smaller ratio (gliding distance) compared to if that same plane unloaded.

@AirCraft Lover this is a good one for thoughts.  Obviously, a brick with tiny wings won't glide as far, but there is a range where weight does not affect (and may even improve) gliding distance.  Remember, weight and altitude are your gliding energy.  This is why they put ballast in gliders.  The heavier glider will glide faster to get sufficient lift at optimal (lift to drag) AOA.  But because it has more energy to start with, it will go as far as the lighter one.  Determination of optimal gliding weight range is a useful and practical study for any design.

I agree that tiny wing will not glide far. What I am saying is that beside the airplane shape (glider of course different than fighter jet in gliding), weight and initial speed or velocity also affecting the glide distance. What I am trying to argue is that L/D is not the only factor as suggested by the link above.

The heaver glider (or airplane) will glide faster. That probably true. But the wight also will affect force factor component. Isn't it?

@Aircraft Lover that is why I love to build and test.  Within a certain range (maybe 1700 to 2300 lbs for a 172?) The glide range is the same as weight/altitude potential energy balance extra drag at higher glide speed.  Too light will lessen range.  Too heavy and AOA (induced drag) from lift requirement also lessens range. So we have a Weight Best Glide or Wbg graph curve too!  This is also known as optimal wing loading.

What kind of airplane did you build? A model? Actually my question is not specific to Cessna 172. My concern is more to glider than to an airplane. But the explanation in the video I provide take Cessna 172 as an example.

@AirCraft Lover works for any size airplane.  Models can be used.  You are correct that higher and higher weight will eventually shorten glide distance.  See SkyBrary.  There is a plateau for Wbg as long as optimal AOA can be maintained. Too light is a concern for wind penetration, hence ballast used on some days with gliders.


text:
How long is maximum time to investigate an airplane crash?
Lion Air flight 610 was crashed on 29 October 2018. It was Boeing 373 Max 8. Almost one year ago. I have not heard any news from National Transportation Safety Committee (NTSC) when they will release the investigation report to media.
From the previous accident, AirAsia QZ8501 occurred on 28 December 2014 with Airbus A320 airplane, NTSC took less than a year time to release the investigation report to media. On 4 December 2015, the NTSC did the release the report.
Then my question is, how long is maximum time to investigate an airplane crash?

The NTSB is not investigating that crash because it occurred in Indonesia. The investigation is carried out by the National Transportation Safety Committee (NTSC) of Indonesia. The NTSB has however sent a representative. They have published a preliminary report so far.

Thank you for the correction about the NTSC. But what I mean is full report, as they did to AirAsia QZ8501's report.

According to ICAO, a preliminary report must be released after a year. There is no hard timeline for the full report:

What are a State’s reporting obligations during and after an aircraft accident investigation?
Under Annex 13 to the Chicago Convention, States in charge of an investigation must submit a Preliminary Report to ICAO within thirty days of the date of the accident, unless the Accident/Incident Data Report has been sent by that time. Preliminary Reports may be marked as confidential or remain public at the investigating State’s discretion.
The State conducting the investigation of an accident or incident shall also make the Final Report publicly available as soon as possible and, if possible, within twelve months.
If the report cannot be made publicly available within twelve months, the State conducting the investigation shall make an interim statement publicly available on each anniversary of the occurrence – detailing the progress of the investigation and any safety issues identified.

Keep in mind there is no established penalty for not complying with the above. Indeed in some cases (or countries) it can take up to a decade for a final report to be published.

From your own quote, it is clearly stated that preliminary report must be submitted within 30 days. an investigation must submit a Preliminary Report to ICAO within thirty days of the date of the accident, and the final report must be made publicly as soon as possible, but maximum within 12 months. the investigation of an accident or incident shall also make the Final Report publicly available as soon as possible and, if possible, within twelve months. So, what I worried about is reasonable as just few more days, the accident will be one year on 29-October 2019 will be one year.

@AirCraftLover it's impossible to set a limit to the duration of an accident inquiry, as it can take very long to get all the information needed. For example a crash off South Africa took years just to get the equipment needed to access the wreckage which was in deep water (and no suitable deep submersibles were available to the investigators). Other accidents may happen in warzones where access is impossible until the fighting ends. Or it may take decades to even find the wreckage (a few years ago a wreck was found dating back to the 1940s for example).

@jwenting, if you read the quoted comment of AEhere carefully, there will be mandatory report within 12 months, or public statement about the investigation's progress.

@AirCraftLover if you read my last paragraph, you'll see that "mandatory" is a meaningless term if there is no way to enforce this mandate. A country can delay in publishing the report, and face little-to-no consequences.


text:
When two pilots are required for a private aircraft, is it a requirement for the PIC to be ATPL?
After discussion on who could be a captain, I would like to clarify, is having ATPL a requirement for PIC on an aircraft that requires two pilots to fly?

Any requirement for one pilot vs two is specified in the aircraft certification, as well at the operating certificate of the owner/operator if they have one, (i.e. part 121 or 135) rather than the licenses of the pilots flying.

Negative.  A person holding a private pilot certificate (at least in the USA) is allowed to add a type rating granting both PIC and SIC privileges in an aircraft type certified for a two person flight crew provided the flights are not carrying passengers or property for compensation or hire.  See 14 CFR 61.117.
An ATPL would be required if said type rated pilot was conducting Part 121 or Part 135 operations in such an aircraft.

So I can be a 747-rated private pilot? Awesome!!

Sure can.  But you may only exercise PPL privileges when operating the aircraft.

An example of not needing an ATP is one pilot flying under the hood doing instrument training, and the 2nd pilot watching for traffic and monitoring approaches perhaps. Both can be Private Pilot, Airplane Single Engine Land, about as minimally rated as it goes (not counting the light aircraft/sport aircraft stuff, as I don't know the airspace limitations on those).